Hot-stamping is a process whereby a transfer of material from a foil onto a thermoplastic surface is achieved by the application of heat and pressure to the foil and plastic. The transfer is generally in the form of a print, or copy, or a design, all of which is determined by the die effecting the transfer. In an actual deposition, the plastic is pressed against the die, sandwiching the foil between the plastic and die, effecting material transfer from the foil carrier to the plastic surface. When the transfer of material to the plastic surface occurs, it is absolutely necessary that the die effecting the transfer and the foil, be in contact with that part of the surface of the plastic which must be printed or decorated. Any lack of contact, no matter how small, will result in no deposit. Since the entire surface to be printed must then be in contact with the die, and under high pressure and high temperature to effect transfer, the surface of the plastic deforms due to to a number of causes:
A. Relief of stresses and strains and, in some cases, memory; PA1 B. Change of state (melt); PA1 C. The introduction of laminar flow of the plastic at a multi-molecular depth from the surface. PA1 A. the plastic composition; PA1 B. the extent (height) and depth of the surface flow; PA1 C. the velocity of movement of the plastic piece past the die (or vice versa). This would be the dwell time of the force applied perpendicularly to the plastic surface; PA1 D. friction, if any; PA1 E. the force due to the momentum of the impact of the plastic piece against the die; PA1 F. the thickness and composition of the piece being hot-stamped; and PA1 G. compliance and/or rigidity of the plastic.
To understand the nature of the surface of molded plastic, it must be realized that when a plastic resin is molded (either by injection or compression, or blow-molded), one does not get a geometrically precise item. In the plastic surface, there are many hills and valleys which occur. These hills and valleys cause numerous problems for hot-stamping operations. Since contact is absolutely essential, it can be readily understood that various ripples and deformations in the plastic surface would prevent surface-to-surface contact. One way of overcoming this problem is to press the die against the plastic with such force that the surface to be printed is essentially equalled out to allow contact. What happens in this case, is that it can only be done safely at low temperatures, so that the types of deformation described previously do not occur; what does occur is, a de-bossing of the plastic surface takes place, not true, permanent, hot-stamping.
It is possible to hot-stamp by vertically pressing a heated die into the plastic, and this is called vertical, or flat, stamping. For using a curved surface (as, for example, a cylindrical bottle or lipstick tube) where more than 25 percent of the surface is to be stamped, then it is a common practice to roll the plastic on a mandrel past a die (flat or curved), and we call this peripheral hot-stamping. The present invention as described is applicable to both flat and peripheral stamping. It is, however, principally directed to peripheral stamping. The most significant difference, however, between the two forms of hot-stamping is that we deposit material over a broad area under a moderate force as quickly as a third of a second with flat stamping; we deposit material in the peripheral type stamping over a very small area under much greater force in, perhaps, 0.005 second, with the described peripheral stamping. We are theoretically printing on a line (a round surface tangentially contacting a flat surface).
These differences become significant when one considers the contribution of the leaf, or foil, to the phenomena of hot-stamping. Theoretically, when the foil material is deposited, it is not only mechanically pressed into the plastic, but should also combine chemically with the plastic being decorated. Thus, temperature, pressure, dwell time and the chemical nature of the leaf, are also much more critical when following a peripheral stamping, as opposed to the flat stamping. In the present invention, flat-stamping and peripheral stamping are both achievable because the adaption of the mechanism to the rheological considerations allows it to function identically in flat stamping as it does when applied to peripheral stamping, albeit peripheral stamping remains much more critical.